1. Field of the Invention
The present invention relates to a transversal filter of the type which has at least one analog shift register.
2. Description of the Prior Art
Transversal filters are known in the art which are constructed with charge-coupled shift devices (CCDs), and employ either the split electrode principle or the CCD possesses parallel outputs at which the signal can be withdrawn. Such devices are described, for example, in the publication "Charge Transfer Devices" by M. F. Tompsett and C. H. Sequin, published by Academic Press, Inc., New York, San Francisco, London, 1975. In these structures the desired filter function is realized as a pulse response, i.e. a short pulse as an input signal produces the desired pulse response as an output signal at the output end of the circuit by dividing the individual electrodes of the CCD in the case of the case of the split electrode principle and by evaluating the parallel outputs in the case of the other principle.
In the split electrode principle, the output signal is formed from the difference between two high voltages which presents considerable circuitry problems. Another problem is that, under the most unfavorable circumstances, small charge parcels are distributed over large areas. In the process employing parallel outputs, floating gate amplifiers are required which, due to their complicated construction, require a multi-layer metalization, for example, and consequently entail problems in production and further problems during operation, for example, as a result of attenuation due to voltage division.
In an application of Karl Knauer et al, Ser. No. 832,232, filed Sept. 12, 1977, and now U.S. Pat. No. 4,163,957, an improved transversal filter is disclosed which has an analog shift register which is provided with a number of parallel inputs and one series output. A further number of individual predeterminable evaluating circuits is provided and each evaluating circuit has at least one signal input for the input of the signal to be filtered and at least one ouput. At its output each evaluating circuit is supplied with a quantity of charge which, in addition to an individual evaluation factor, is equal to the difference quantity of the relevant signal value of the signal to be filtered and a predetermined minimum value which is smaller than or equal to the value of the signal minimum, or apart from the individual factor is equal to the difference quantity of a predeterminable maximum value which is greater than or equal to the value of the signal maximum and the relevant signal value. The output of each evaluating circuit can be connected by way of a switching element to an associated parallel input and the capacity of each storage position of the shift register is at least such that it can always accommodate the maximum quantity of charge supplied by the adjacent storage position in the direction opposite to that of the shift process, and when the storage position is one having a parallel input, the capacity can additionally accommodate the maximum quantities of charge supplied by the associated evaluating circuit or circuits. In a preferred embodiment of that invention, each parallel input is assigned only one of the evaluating circuits.